villanova university dept. of civil & environmental engineering cee 4606 - capstone ii...

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Villanova UniversityDept. of Civil & Environmental Engineering

CEE 4606 - Capstone IIStructural Engineering


Lecture 5 – Gravity Load Design (Part 1)

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Outline

1. Review of Progress Report #1 Presentations

2. IBC Concrete Design Requirements3. Beam & One Way Slab Design4. Slab Thickness Considerations5. Load Path and Framing Possibilities6. Connection & Analysis Issues7. Seismic Detailing Requirements8. Work Tasks

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Progress Report #1 Comments

• Overall, a very good job• Comments on presentations:

– Timing good– Don’t worry about the intro stuff next

time– Know where our site is located – you

have coordinates that are accurate to within 3 miles!!!

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Progress Report #1 Comments

• Range of values:– 100 to 150 mph design wind speed– Seismic Design Category D

(unanimous)– 2000 to 2800 psi concrete strength– 49000 to 53400 psi steel yield

strength

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IBC Concrete Design Requirements

• IBC Chapter 19• Mimics ACI 318 Code

– IBC 2000 version based on 1999 ACI 318– IBC 2003 will use 2002 version of ACI

318

• First seven sections (1901 – 1907) correspond to ACI 318 Chapters 1 to 7

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IBC Concrete Design Requirements

• Section 1908 gives specific modifications to ACI 318– Deals with “meat” of ACI Code

• Sections 1909 – 1916 deal with specialized areas– Sec. 1910 – Seismic Design

Requirements– Sec. 1912 – Anchorage to Concrete

• Get to know this document!!!

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Load Path / Framing Issues

• Building Frame System– Frame for gravity load– Shear walls for lateral load

• Consider support of the chapel gravity loads:– Where do the columns go?– What beams do I need?– How do I design my slab?

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Beam & One Way Slab Design Review

• We presumably know how to do the following from CEE 3422:– Design a rectangular beam of

unknown cross-section size– Design a rectangular beam of known

cross-section size– Design a simply supported one way

slab

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Beam & One Way Slab Design Review

• We presumably know how to do the following from CEE 3422:– Design a T-beam for positive moment– Design a T-beam for negative moment– Design a doubly reinforced beam

(beam with compression reinforcement)

– Design a beam for shear

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Design of Continuous Beams and Slabs

• You know how to design cross-sections for positive or negative moment

• Reinforcement follows the moment diagram

• Why continuous spans?– Moments – Deflections

Two Simple Spans

Continuous over Center Support

Gap

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Design Moments (Uniform Dist. Loading)

• Simple Spans– wL2/8

• Continuous Spans– Analysis far more complicated– What type of fixity do we actually have?– Must consider effects of patterned

loading– Formation of plastic hinges allows for

moment redistribution

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Design Moments – Continuous Spans

• We have four analysis options– Elastic Analysis (preferably STAAD)– Elastic Analysis w/ Moment

Redistribution– Approximate Frame Analysis– ACI Approximate Moment Coefficients

• See McCormac text Chapter 13

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Slab Thickness Considerations

• What governs the thickness of a slab?– Flexural Strength– Shear– Deflections

• Usually, deflections will govern the thickness requirements for a one-way slab– Size slab based on deflection requirements– Check shear– Design reinforcement for flexure

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• Review McCormac text, Ch. 5 (serviceability) and Ch. 3 (one-way slabs)

• Review notes from CEE 3422, lectures on one-way slab design and serviceability

• ACI Sec. 9.5.2.1

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Slab Thickness Considerations(such that we do not need to compute

deflections)• For simply-supported beams, total beam

depth ‘h’ must be at least L/16– A 16 ft. long simply supported beam must be

at least 12 in. deep.

• For simply-supported one-way slabs, total slab thickness ‘h’ must be at least L/20– A 10 ft. long simply supported one-way slab

must be at least 6 in. deep.

• You will have to look up other values!!!

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• Something to keep in mind….– Your material properties!– These tables are based on normal

strength concrete– You may wish to consider creative

ways to adjust tables for your low concrete strength• Hint: Think about what the key concrete

material property related to deflections is…

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Load Path / Framing Possibilities

• Now we can begin to develop a framing plan for our structure– Typical practice on site is a 5 in. thick slab– We have a methodology to determine how

far a slab of a given thickness can span– Do our material properties have any effect?

• Let’s look at a plan view of the two-story section…

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Ln = 10.5 ft.

Ln = 12.0 ft.

Ln = 14.5 ft.

Ln = 27.0 ft.

Think we’ll need some additional framing members???

Note: columns automatically placed at each wall end or corner

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Framing Concepts

• Let’s use a simple example for our discussion…

• Column spacing– 30 ft. on center

• Think about relating it to your design as we discuss…

Plan

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Framing Concepts

• We can first assume that we’ll have major girders running in one direction in our one-way system

Plan

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Framing Concepts

• If we span between girders with our slab, then we have a load path, but if the spans are too long…

Plan

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Framing Concepts

• We will need to shorten up the span with additional beams

Plan

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Framing Concepts

• But we need to support the load from these new beams, so we will need additional supporting members

Plan

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Framing Concepts

• Now we have a viable plan…

• Let’s think back through our load path now to identify our “heirarchy” of members

Plan

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Framing Concepts

• One-Way Slab (continuous)

• Beams– Interior (T-beams)– Exterior (L-beams)

• Girders– Interior (T-beams)– Exterior (L-beams)

Plan

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Framing Concepts

• Note that by running the one-way slab in this EW direction, we are actually making the EW running beams our major girders

• The NS running beams simply transfer the load out to these girders (or directly to a column) Plan

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Framing Concepts

• Now let’s go back through with a slightly different load path

Plan

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Framing Concepts

• We again assume that we’ll have major girders running in one direction in our one-way system

Plan

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Framing Concepts

• This time, let’s think about shortening up the slab span by running beams into our girders.

• Our one-way slab will transfer our load to the beams. Plan

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Framing Concepts

• With this approach, we have already established our “heirarchy”

• The only difference is in the “direction” of our load path– 90 degree rotation

Plan

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Framing Concepts - Conclusions

• Either load path will work• In this case, they are identical• With a rectangular bay (instead of a

square) bay, there will be a difference

• Tradeoff is usually in number of supporting members vs. span of supporting members

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Two Load Path Options

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Framing Concepts - Considerations

• For your structure:– Look for a “natural” load path– Identify which column lines are best

suited to having major framing members (i.e. girders)

– Assume walls are not there for structural support, but consider that the may help you in construction (forming)

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Connection / Analysis Issues

• With continuous reinforced concrete framing systems, connections are a major issue with respect to:– Detailing of reinforcement at these

congested areas– Assumptions regarding fixity of

beams and slabs

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• Let’s first consider our continuous one-way slab (12” strip shown) framing into an exterior (spandrel) beam

Plan

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Slab-Exterior Beam Connection

• Slab is a six span continuous system• Some fixity at end of slab due to torsional

rigidity of exterior beam, but what happens when beam and slab crack?– Do we want to count on fixity?

• Also, if we design slab for negative moment here, we must develop reinforcement (like a cantilever)

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Typical assumptions:– Simple support

at end– No moment in

slab at end– Place some

reinforcement at top of slab to control cracking

– Design exterior beam for minimal torsion

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• Now let’s consider our beam-girder joints

Plan

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Beam-Girder Connection

• Beam is a two span continuous system• Similar situation: some fixity at end of beam

due to torsional rigidity of exterior girder, but what happens when beam and girder crack?– Do we want to count on fixity?

• Also, if we design beam for negative moment here, we must develop reinforcement (like a cantilever)

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Typical assumptions:– Simple support at

end– No moment in

beam at end– Place some

reinforcement at top of beam to control cracking

– Design exterior girder for minimal torsion

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Analysis – One-Way Slab & T-Beams

• For the simple elements just described, where supports are provided by beams and girders,– Supporting elements have some stiffness,

but it is fairly small– Assumption of treating one-way slabs and T-

beams as continuous beams is valid– A frame analysis is not needed since there

are no columns involved– Simple analysis methods can be used if all

assumptions are met (i.e. ACI moment coefficients)

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• Finally, let’s look at beam-column and girder-column joints

• Three situations:– Interior column– Exterior column– Corner column

Plan

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Interior Column Connection

• Girders framing in to a column:– Columns will provide some rigidity – Moments will depend upon

distribution of stiffness– Frame analysis is warranted to

determine these moments– Unbalanced loading (patterned

live load) must be considered– Goal: Determine moments in

girders (they will not necessarily be equal), as well as axial load & moment combinations for columns

• Beam/girder reinforcement must be continuous through joint

Plan

cl

cu

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Exterior Column Connection

• Same basic situation:– Columns will provide some rigidity – Moments will depend upon

distribution of stiffness– Frame analysis is warranted to

determine these moments– Unbalanced loading (patterned live

load) must be considered– Goal: Determine moments in

girders (they will not necessarily be equal), as well as axial load & moment combinations for columns

• Beam/girder reinforcement must be developed for negative moment

Plan

cl

cu

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Corner Column Connection

• This is essentially the same situation as an exterior column

• Note that where we have beams (not girders) framing into columns, the same principles apply– However, these moments are

typically very small and will usually not control the design

cl

cu

Plan

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Analysis – Girders & Beams Framing Into Columns

• For these elements, support is provided by columns

• Columns have substantial stiffness and will attract some moments– Assumption of treating these girders and

beams as continuous beams is not valid– A frame analysis is needed to determine the

appropriate distribution of moments– Elastic analysis is recommended (STAAD,

PCABeam)

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Seismic Detailing Requirements for Reinforced Concrete - Introduction

• IBC Section 1910• ACI 318-99 Chapter 21• These two sections, together,

identify specific detailing requirements related to seismic design of concrete structures

• Level of detailing required is based on Seismic Design Category

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Work Tasks

• Determine final loads on the structure– Gravity loads (dead, live)– Lateral loads (seismic, wind)

• Truss analysis on roof & design of roof members

• Detailing of roof-to-structure connection• Develop a load path (framing plan) to

support the gravity loads associated with the second story chapel

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Work Tasks

• Look into how the selection of Seismic Design Category D will affect concrete design detailing requirements for your beams, columns, and slab

• Work on design of one-way slab, beams, and girders– We will discuss design for shear and torsion

next time!

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Assignment for Tuesday

1. Submit a detailed sketch showing your framing plan (load path for gravity loads) for the second story chapel– Identify all columns, beam, and girder locations, and

specify a slab thickness

2. Summarize on one sheet how the selection of Seismic Design Category D will affect the detailing of your structure– Use a bullet item / list format to identify specific

detailing requirements for your beams, columns, and slab

– Don’t consider shear walls for now (they will be masonry)

villanova university dept. of civil & environmental engineering cee 4606 - capstone ii...

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